1. Field of the Invention
The present invention relates to a novel fiber of a fluorocarbon polymer and a process for producing the same. More particularly, the present invention is concerned with a novel fiber of a fluorocarbon polymer, which not only has ion exchange properties, swelling properties, shrinking properties, and resistance to heat and corrosion but also has high tensile strength at break and which, therefore, is useful for various applications such as recovery of heavy metals, detection of humidity change and measurement of salt concentration and can also be employed as a reinforcing material for films, membranes, etc. The present invention is also concerned with a process for producing such a fiber by preparing a filament from a fluorocarbon polymer having ion exchange precursor groups in melt-fabricatable form, converting the precursor groups of the polymer filament to ion exchange groups in melt-nonfabricatable form, and then drawing the resultant melt-nonfabricatable filament at a temperature within a specific range.
2. Discussion of Related Art
Fibers of a fluorocarbon polymer having ion exchange properties are known. For example, U.S. Pat. No. 3,985,501 discloses a fiber of a fluorinated polymer containing sulfonyl groups, and in this patent, it is described that the sulfonyl groups are in the form of sulfonamide groups, sulfonic acid groups or a salt thereof. Further, U.S. Pat. No. 3,940,916 discloses a woven or knitted fabric comprising filaments of a fluorinated polymer containing sulfonyl groups, the filaments having a size of not larger than 400 denier and being individually supported by a high strength reinforcing material.
As disclosed in the above-mentioned patents, in general, a fiber having ion exchange properties is produced from a thermoplastic polymer containing ion exchange precursor groups using a customary melt spinning technique. The customary melt spinning technique includes drawing a spun filament in which the spun filament is generally drawn by 50 to 400%. However, even by such drawing, the strength of the filament cannot be satisfactorily improved and, therefore, it is difficult to perform fabrication, for example, weaving of the filament without occurrence of breaks of the filament. Therefore, as disclosed in U.S. Pat. No. 3,940,916, it is inevitable that the filaments are supported by a high strength reinforcing material. The use of a supporting high strength reinforcing material is disadvantageous because the need for such reinforcing material is only temporary for performing the weaving operation and the reinforcing material does not contribute to the function of the resultant woven fabric. In addition, the use of reinforcing material disadvantageously causes the weaving operation to be cumbersome.
Further, Japanese Patent Application Publication No. 60-40459 discloses an ion exchange membrane reinforced by a woven fabric obtained by weaving a fiber having ion exchange groups and another fiber having no ion exchange groups. The above-mentioned Patent Application Publication contains no description about the process for producing the fiber having ion exchange groups and, therefore, it is considered that a customary melt spinning technique is employed, which means that this prior art fiber also has the same problem with respect to the strength as mentioned above.